1. Field of the Invention
The present invention relates to an optical barrier apparatus (which is so called "an optical barrier sensor") such as used for industrial security equipment and so on.
2. Brief Description of the Prior Art
An optical barrier apparatus is basically composed of a pair of a light projector and a combined light receiver. The apparatus is to detect an absence of substance between the light projector and the light receiver by sensing that an optical beam projected from the light projector is directly incident to the light receiver. Patents concerning fail-safe optical barrier sensors include PCT International Publication Numbers WO93/23772 and WO95/10789 and are well-known.
If any apparatus is utilized as industrial security equipment, the apparatus has to have a fail-safe constitution. Accordingly, the apparatus is constituted so as to transmit an output signal indicating the absence of the substance only when the optical beam arrives at the light receiver. On the contrary, the apparatus is constituted so as to propagate another output signal for indicating the existence of a substance when the substance actually exists between the light projector and the light receiver as well as when any failure arises in the apparatus.
One light projector and one combined light receiver is sufficient for detecting the existence of the substance in some cases. However, there are many cases wherein the region to be monitored should be broadened. Those cases are investigated below.
A first procedure to be considered is to broaden the region to be detected by employing plural pairs of the light projectors and the light receivers. In order not to endanger the fail-safe property of the system during broadening, each pair, composed of one light projector and one light receiver, should project and receive only one optical beam modulated with a certain specified frequency to avoid an interference with other optical beams. An optical barrier apparatus fabricated according to this first procedure is illustrated as Conventional Example 1 as shown in FIG. 24.
A second procedure to be considered is to scan the region to be inspected with the lights to detect whether the substance is present or not by sensing the cut-off status of the optical beams. An optical barrier apparatus, wherein the fail-safe property is maintained by a use of this second procedure, is illustrated as Conventional Example 2 as shown in FIG. 25. The light projections and the light receivers are constituted to be operated synchronously to avoid interferences while the region to be inspected is scanned with the optical beams which are irradiated sequentially from B1 to Bn.
In an asynchronous type sensor, such as aforementioned Conventional Example 1, many sorts of band-pass filters having different passing frequencies differing from each other are required so the manufacturing cost is high because of an increase in species number of components. Although the resolution in the detecting system is decided by the physical alignment pitch of the light projectors and the light receivers, the alignment pitch cannot be reduced much. LEDs (light emitting diodes) are usually employed as the light projectors because they are the most economical when many light projectors are required. The rays of light emitted from the LED diverge, unlike a laser beam. Accordingly, to converge the rays of light emitted from the LED into one optical beam requires a lens. The light receiver also requires a lens for collecting the rays of light, which cannot shrink the alignment pitch. These lenses account for a major part of the manufacturing costs and therefore a reduction in manufacturing costs of this type of equipment is difficult. Further, because the diameter of the lens is preferably large to improve the optical disturbance-induced S/N ratio. Detection of a signal light generated from the light projector is impossible if the optical power of ambient light is more intense than that of the signal light projected from the light projector. A poor S/N ratio makes it impossible to carry out the system into practice. Consequently, a certain minimum aperture for the lens is necessary, which adversely effects the resolution of a target.
In the synchronous type sensor, such as shown in aforementioned Conventional Example 2, it is designed such that an output signal is not generated if the synchronous circuit fails. This approach is intended to attain the fail-safe characteristics but renders the circuitry complex. Namely, employing only one sort of band-pass filter reduces the manufacturing cost but the fail-safe synchronous circuit greatly increases the manufacturing costs. Alternatively, a great number of pairs composed of the light projectors and the light receivers may be used in a time-sharing manner but this approach includes the same defects as those the asynchronous type has. Namely, the resolution decided by the aperture of the lens has a certain limitation while the number of components, such as lenses, is so high that any reduction in manufacturing cost remains difficult.